Elsevier

The Lancet

Volume 368, Issue 9541, 23–29 September 2006, Pages 1106-1118
The Lancet

Seminar
Human schistosomiasis

https://doi.org/10.1016/S0140-6736(06)69440-3Get rights and content

Summary

Schistosomiasis or bilharzia is a tropical disease caused by worms of the genus Schistosoma. The transmission cycle requires contamination of surface water by excreta, specific freshwater snails as intermediate hosts, and human water contact. The main disease-causing species are S haematobium, S mansoni, and S japonicum. According to WHO, 200 million people are infected worldwide, leading to the loss of 1·53 million disability-adjusted life years, although these figures need revision. Schistosomiasis is characterised by focal epidemiology and overdispersed population distribution, with higher infection rates in children than in adults. Complex immune mechanisms lead to the slow acquisition of immune resistance, though innate factors also play a part. Acute schistosomiasis, a feverish syndrome, is mostly seen in travellers after primary infection. Chronic schistosomal disease affects mainly individuals with long-standing infections in poor rural areas. Immunopathological reactions against schistosome eggs trapped in the tissues lead to inflammatory and obstructive disease in the urinary system (S haematobium) or intestinal disease, hepatosplenic inflammation, and liver fibrosis (S mansoni, S japonicum). The diagnostic standard is microscopic demonstration of eggs in the excreta. Praziquantel is the drug treatment of choice. Vaccines are not yet available. Great advances have been made in the control of the disease through population-based chemotherapy but these required political commitment and strong health systems.

Section snippets

Acute pathology

The percutanous penetration of cercariae can provoke a temporary urticarial rash that sometimes persists for days as papulopruriginous lesions, especially after primary infections such as occur in tourists and migrants.15 A similar swimmers' itch is also frequently caused by cercariae of animal trematodes in temperate climate zones.16 Possibly, cercarial dermatitis often goes unrecognised in endemic areas.17

Acute schistosomiasis (Katayama fever) is a systemic hypersensitivity reaction against

Chronic pathology and morbidity

The main lesions in established and chronic infection are due not to the adult worms but to eggs that are trapped in the tissues during the perivesical or peri-intestinal migration or after embolisation in the liver, spleen, lungs, or cerebrospinal system. The eggs secrete proteolytic enzymes that provoke typical eosinophilic inflammatory and granulomatous reactions, which are progressively replaced by fibrotic deposits (figure 4).26 The severity of the symptoms is thus related both to the

Diagnosis

The microscopic examination of excreta remains the gold standard for the diagnosis of schistosomiasis.71 The eggs are easy to detect and identify by microscopy owing to their size and shape, their typical lateral or terminal spine, and the living miracidium (in fresh samples) with mobile cilia and pulsing excretory cells (figure 5). Direct wet slides are not very sensitive; if no eggs are found, concentration methods should be used but even these can miss light infections.72, 73

Urine should be

Treatment

Early treatments against schistosomiasis had severe and even lethal side-effects that had to be weighed against the benefits for the patient.4, 5 The 1970s heralded the advent of effective, safe, and simple drugs.34

Praziquantel, an acylated quinoline-pyrazine that is active against all schistosome species, is now the most widely used. It is mostly marketed as 600 mg tablets, with a recommended standard regimen of 40 mg/kg bodyweight in a single dose.6 The drug acts within 1 h of ingestion by

Immunology

There is longstanding epidemiological and clinical evidence that people living in endemic areas acquire some form of immune resistance after years of exposure.113 In terms of parasite population dynamics, host-related factors such as innate or acquired immunity are likely to have an important role in truncating the enormous reproduction potential of schistosomes to the endemic equilibrium of one.114 The acquisition of effective immunity is difficult to prove, because the decrease in infection

Global burden

Schistosomiasis is highly prevalent, but the associated morbidity is low and variable. Thus, its influence on public health and the priority of control measures have long been debated. The discussion has been revived in light of the renewed resources for the fight against poverty-related diseases and the Global Burden of Disease Study, which attempted to quantify and rank health problems according to disability-adjusted life years (DALY).133 This index is calculated from disease-specific

Control

The aims and strategies of schistosomiasis control have shifted fundamentally over the past few decades, since the introduction of modern schistosomicides, particularly praziquantel. As for other parasitic diseases, transmission control aiming at the intermediate host has been largely replaced by morbidity control through population-based chemotherapy. This strategy allows quick gains, but careful long-term planning is needed to ensure sustainability and progression to the more demanding stages

The way forward

In theory, doctors and other health workers have adequate tools at hand for diagnosis and treatment of most overt cases of schistosomiasis in outpatient or primary care. However, detection of light infections and assessment of their clinical importance remains more difficult. Resistance to praziquantel should be avoided at all costs, and new drugs would be welcome. Improved diagnostic agents and therapeutic strategies are therefore main topics for further applied research on schistosomiasis. A

Search strategy and selection criteria

The literature research for this Seminar started from standard works and recent reviews.1, 2, 3, 4, 5, 6, 7, 8, 9, 10 We also used our own collections and the library holdings of the Antwerp Institute of Tropical Medicine, and searched PubMed and MEDLINE over the past 10 years for the main topics of this paper with the key words “schistosomes OR schistosomiasis OR schistosoma” PLUS “epidemiology”, “transmission”, “pathology”, “morbidity”, “mortality”, “immunology”, “vaccine”, “treatment”,

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